Ink compositions for printing patterns of conductors or resistors on ceramic substrates, in the manufacture of thick-film hybrid circuits, have usually comprised pastes containing certain noble metals and noble metal oxides, glass frits, organic binders and solvents. Usually, the pastes have been screen-printed on the substrate and fired to sinter the metallic ingredients, fuse the glass components and burn off the organic binders. The glass components as well as certain other ingredients such as bismuth oxide and other fluxing agents, are present to secure adhesion to the substrate. This type of composition is known as a frit-bonded type.
Because the frit-bonded type compositions have usually included rather high proportions of very expensive metals such as platinum, palladium and gold, an effort has been made to find suitable ink compositions utilizing less expensive metals. Also, because compositions containing substantial percentages of glass frits i.e. 5-30 weight %, are sometimes difficult to make soldered connections to, an effort has been made to discover compositions which use little or no glass frit but have good adherence properties and high conductivity (where conductors rather than resistors are involved).
The effort to find suitable substitutes for frit-bonded ink compositions has resulted in the discovery of another type of ink known as a reactively-bonded ink. In this type of ink, adherence to the substrate takes place principally through the presence of discrete chemical compounds which are formed during firing, at the film-substrate interface, from components of both the thick-film ink and the substrate.
Reactively-bonded inks contain little or no glass frit. They do contain metals and non-vitreous metal oxides, organic binders and solvents and they may also contain other agents to promote adherence to the substrate. Previously, reactively-bonded inks have used mostly heavy base metals such as molybdenum, manganese and tungsten and also iron, chromium, nickel and thorium. These metals have provided strong, age-resistant bonds but they all oxidize and some, like molybdenum, have volatile oxides so that they cannot be fired in air. These materials also usually are not directly wettable by solders (although nickel is an exception to this).